Combined electron microscopic and computer studies will be used to obtain information on the type of structures which are involved in the folding of single-stranded RNAs from the bacteriophage MS2, poliovirus and T7 mRNA. The work has been facilitated by the recent development of a high speed computer program for predicting loops from sequence data on large molecules. Improved electron microscopic mapping data will be obtained by hybridizing restriction fragments of cloned MS2 DNA to MS2 RNA. The hybrid molecules will be examined for the loss of loop structures in the electron microscope.